Apparatuses for detecting and measuring the presence and level of fluid in a given medium are commonly used for monitoring and controlling fluid levels, flow, relative moisture contents, liquid spills, contamination and the like.
Contemporary sensor technology depends on changes in the dielectric property of the medium caused by variances in fluid level or fluid concentration within the medium. Sensors of contemporary design typically utilize a parallel plate capacitor immersed or embedded in the medium so that a portion of the medium may function as a dielectric between the plates. The capacitor is part of the RC circuit of an oscillator whose frequency varies with changes in the dielectric property of that portion of the medium between the plates. This technique is generally satisfactory when the dielectric-forming medium is a fluid; the gap between the plates can be narrow enough to yield a relatively high capacitance. However, when dealing with granular or pulverous media, a wider plate gap is necessary; and inaccuracies can be introduced by changes in the enclosed medium. Furthermore, as the fluid absorbing medium becomes increasingly saturated, the relationship between the dielectric value and fluid concentration becomes less linear. As a consequence, accuracy of fluid concentration measurement suffers.
Another problem of relying on changes in the dielectric properties of a surrounding medium is that impurities contained in the water produce significant changes in the dielectric constant which are not easily determined. Thus, the presence of salts will produce the same effective capacitance as a larger volume of water.
By accurately monitoring moisture levels in cultivated land in order to provide optimum irrigation, significant gains can be achieved in water and energy conservation. The control of sprinklers, pumps and other irrigation equipment from moisture monitors embedded at plant root-level has almost become a necessity in the water-poor sun-belt western states.
Conventional moisture sensors of the type described above are not only inaccurate under certain conditions, but are difficult to embed in the ground due to their fragile plate configuration and internal electronic components. There is a need for a more sensitive type of soil moisture monitor packaged in an easily embedded, durable, compact enclosure.